There are a number of methods in the prior art for developing latent electrostatic images. These images may be formed by charging a photoconductive surface and then exposing the charged surface to light or ionizing radiations adapted to render the photoconductor conductive in the areas exposed to light or radiant energy and thus permit charges on the surface to be conducted to ground, in whole or in part. The latent electrostatic image may be formed on a dielectric sheet by energizing styli to form the desired electrostatic pattern on the surface of the dielectric sheet. The development of latent electrostatic images was first accomplished by the electrical attraction of fine particles to the charged areas forming the latent electrostatic image. Usually, these particles carried electrical charges opposite in polarity to the charges forming the latent electrostatic image. These particles were usually dry, from which the process derived the name "xerography". Later, it was discovered that particles could be suspended in a dielectric liquid and the latent electrostatic image could be developed by immersing it in the liquid. The particles moved through the liquid by electrophoresis, going to the charges of the image and developing it. The particles, of course, were visible so the image would become apparent.
When dry powder was used as the developing medium, it was usually transferred to plain paper or other carrier sheet and then fixed by fusing, the particles in this case, of course, being thermoplastic. This required heat to melt the heat-fusible resin forming or associated with the dry particles. If a carrier sheet, such as paper, was coated with a photoconductive material, such as zinc oxide, no transfer of the developed electrostatic image was required where the developer was a liquid carrying toner particles.
As the art developed, it was convenient, in order to eliminate the necessity of fusing a powder-developed image, to develop the latent electrostatic image with a liquid developer and then transfer the developed image to plain paper. The developer liquid usually comprised a hydrocarbon carrier, such as Isopar-G, or the like, through which were dispersed toner particles adapted to render the latent electrostatic image visible. The immersing of the photoconductive surface carrying the latent electrostatic image in a bath of liquid developer required wetting the entire surface of the photoconductor. The amount of liquid developer remaining on the developed image could be largely removed by a doctor roller. The remaining liquid, along with the developed image, would be transferred to paper, or other carrier, and had to be dried on the paper. This resulted in continual evaporation of a small amount of carrier liquid transferred from the photoconductive surface to the carrier paper. This evaporation was undesirable from several standpoints: First, the necessity of drying the sheet not only required energy, but the time involved in such operation limited the speed of copying. Furthermore, the ratio of liquid carrier to toner would constantly vary, depending on the amount of toner particles employed in the development process.
In the prior art, a stationary applicator adjacent the photoconductor was usually employed. In order to obtain complete development, the spacing between the stationary applicator and the photoconductor which was carried by a drum moving past the applicator was in the order of twenty mils. This large gap was necessary because it was exceedingly expensive to provide bearings for a drum which would ensure that there would be no contact between the photoconductor and the stationary applicator electrode. The mechanical precision involved in using a close clearance would increase the cost of manufacturing to an unacceptable point. The supplying of liquid developer to the gap between the stationary electrode and the rotary photoconductor ensured that the entire photoconductor became wetted with the developer liquid.
The art also attempted to overcome the difficulties experienced in employing an applicator roller in contact with the drum by using a compressible or resilient roller, formed with sponge-like material, which would apply liquid to the latent electrostatic image and then, as the pressure was released, suck the excess liquid from the photoconductive surface.
It was also attempted to overcome the difficulties experienced in employing an applicator roller by separating the applicator roller from the photoconductive surface and permitting the electrostatic field of the latent electrostatic image to pull the developing liquid from the applicator roller.